1. Field of the Invention
The present invention relates to a data repeating device that receives and forwards network data signals at the physical layer, a data communications device that processes network data at the data link layer or above, and a data communications system including those apparatuses. More particularly, the present invention relates to a data repeating device, a data communications device, and a data communications system having a mechanism for enhanced data integrity.
2. Description of the Related Art
Recent years have seen an increased number of devices with a local area network (LAN) interface for use with Internet Protocol (IP) networks, not only for computer communications, but also for voice communications such as IP telephony services. Those IP network devices (or IP devices) are broadly classified into two categories: intelligent devices having a central processing unit (CPU) to implement sophisticated functions with software programs, and CPU-less devices without software control. IP devices in the former category, or software-controlled devices, include switching hubs, IP routers, media converters, computer network interface cards. Generally, those devices have the functions of processing packets at the data link layer or above. IP devices in the latter category, on the other hand, handle network signals only at the physical layer to repeat (i.e., receive and forward) packets. Such devices include simple media converters designed only for the transport of main data signals.
Network systems are often structured in a hierarchical manner, from access networks to trunk networks. Bit error rate requirements in a particular network depend on where in the system hierarchy that network is. Generally a network located closer to the core network (i.e., trunk transmission facilities interconnecting communications carriers) is supposed to be more reliable and stable. With the increased use of both types of IP devices mentioned above, the requirements for their service quality will also increase up to a level comparable to Synchronous Optical Network (SONET) or Synchronous Digital Hierarchy (SDH).
Because of their enhanced control capabilities, software-controlled IP devices fulfill such service requirements by taking advantage of various measures including bit error monitoring, forward error correction (FEC), and dynamic routing control. By contrast, software-less physical-layer network devices are designed primarily for transmission of main data signals, without putting much consideration into bit errors or the like.
FIG. 24 shows a first example of a conventional data communications system. This system is an IP network using physical-layer media converters. Specifically, two layer-2 (L2) switches 911 and 914 are interconnected via CPU-less media converters 912 and 913, where the media converters 912 and 913 convert data signals at the physical layer (i.e., optical to electrical, and vice versa).
The leftmost L2 switch 911 and media converter 912 are interconnected via an Ethernet (registered trademark of Xerox Corporation) connection using an unshielded twisted pair (UTP) cable. This part is referred to as a LAN link 915. Likewise, the rightmost L2 switch 914 and media converter 913 are interconnected via a LAN link 916 with a UTP cable. The two media converters 912 and 913 are interconnected by a fiber-optic link 917. For this optical connection, the media converters 912 and 913 have single-fiber bidirectional optical modules (not shown).
Specifically, the above data communications system operates in the following way:                (a) The L2 switch 911 transmits a normal frame 921, which is received by a media converter 912.        (b) The media converter 912 retransmits the received normal frame 922 to another media converter 913. It is supposed here that the frame 922 suffers an error on the fiber-optic link 917 for some reason.        (c) The media converter 913 receives a distorted frame 923. The media converter 913 thus delivers it as a distorted frame 924 to the L2 switch 914.        (d) The L2 switch 914 discards the received frame 924 because its data is corrupted.        
FIG. 25 shows a second example of a conventional data communications system. This system includes media converters 931 and 932 that operate at the physical and data link layers, based on the TS-1000 specifications standardized by Telecommunication Technology Committee (TTC) in Japan for Fiber-to-the-Home (FTTH) applications. The media converters 931 and 932 are interconnected by a fiber-optic link 935, besides being coupled to their nearest LAN segments via LAN links 933 and 934, respectively. Frames 936 are transported through those media converters 931 and 932.
The two media converters 931 and 932 in the system of FIG. 25 play different roles, the former as a center-side device and the latter as a terminal-side device. The center-side media converter 931 has a CPU to control the terminal-side media converter 932. The CPU performs, for example, a loopback test to detect failure. The center-side media converter 931 remotely monitors the state of the terminal-side media converter 932, including its link fault, power disruption, and hardware failure. Specifically, the center-side media converter 931 sends a supervisory frame (“Request”) to the terminal-side media converter 932 and checks whether the terminal-side media converter 932 returns a supervisory response frame (“Response”).
The center-side media converter 931 can also cause the terminal-side media converter 932 to generate an alert (e.g., packet error alert). Yet another function of the center-side media converter 931 is to remotely configure the terminal-side media converter 932.
FIG. 26 shows a third example of a conventional data communications system. This system uses error correction functions (e.g., FEC and Read-Solomon coding) for reliable transmission of frames between distant L2 switches 941 and 944. The two L2 switches 941 and 944 are interconnected via CPU-less media converters 942 and 943 that process data signals at the physical layer.
In FIG. 26, the leftmost L2 switch 941 and media converter 942 are interconnected via a LAN link 945. Similarly, the rightmost L2 switch 944 and media converter 943 are interconnected via a LAN link 946. Two media converters 942 and 943 are interconnected via a fiber-optic link 947. For this optical connection, the media converters 942 and 943 have single-fiber bidirectional optical modules.
For example, the data communications system of FIG. 26 operates in the following way:                (a) The L2 switch 941 transmits a normal frame 951, which is received by a media converter 942.        (b) The media converter 942 retransmits a normal frame 952 to another media converter 943. It is supposed here that the frame 952 suffers an error on the fiber-optic link 947.        (c) The media converter 943 thus receives a distorted frame 953. The media converter 943 retransmits it intact as a distorted frame 954 to the L2 switch 944.        (d) The L2 switch 944 recovers the original transmission data from the received frame 954 by correcting its error with the FEC algorithm that is implemented.        
The use of error correction techniques enables the receiving device (i.e., L2 switch 944 in the present example) to correct errors that may be introduced to the transmitted frames due to a failure of the transmission line or network device. Various data communications systems with FEC have been proposed. See Japanese Unexamined Patent Publication 2001-45098, for example.
It should be noted here that media converters and other packet repeating devices operating only at the physical layer simply forward received packets to subsequent devices, whether their data is corrupted or not. Data errors can primarily be categorized into two groups: those produced on a transmission line and those produced in a network device. The causes of errors in the former group include transmission line loss, poor fiber bandwidth characteristics, and noises. The errors in the latter group range from temporary, recoverable errors (e.g., synchronization error) to permanent, unrecoverable errors (e.g., hardware failure).
With respect to the issue of data errors mentioned above, the system using physical-layer media converters illustrated in FIG. 24 has the following drawbacks:                (a) Packet errors can only be detected by upper-layer devices such as switching hubs. But they are unable to identify the failure location (e.g., on a transmission line, inside a media converter, or anywhere else).        (b) Corrupted frames are allowed to travel over the transmission line until they are terminated at a layer-2 device.Therefore, the system of FIG. 24 cannot locate the cause of packet errors.        
In the network system discussed in FIG. 25, error monitoring functions can be implemented since the system includes an intelligent media converter having a CPU. This system, however, has the following drawbacks:                (a) Corrupted frames are allowed to travel over the transmission line until they are terminated at a layer-2 device.        (b) While being able to monitor the number of corrupted packets and total transmitted packets, the media converter provide no further functions such as selecting new links or correcting errors.        (c) The cost of the center-side media converter is inevitably increased because of the use of a CPU for software-based control capabilities.        (d) The supervisory functions are intended for an optical link between a central device and a terminal device, as in an FTTH network. LAN links, on the other hand, are out of the coverage.        
The system of FIG. 26 uses error correction coding to improve the resilience to frame data errors. However, this system has a drawback in that every frame has to carry an additional code for error correction even in normal circumstances where no errors are encountered on the transmission line. Transmitting those extra bits reduces the efficiency of data transmission.
As can be seen from the above discussion, network systems need a mechanism of correcting errors automatically even if frequent errors are produced due to, for example, a poor bandwidth of fiber optic media. While the use of error correction coding is an effective solution for this demand, conventional systems sacrifice the efficiency of data transmission in normal conditions, giving higher priority to the data integrity in error-prone circumstances.